The latest addition to knowledge of the Solar System looks a bit like a couple of potatoes that have lain together and dried over several years. It also has a name – Arrokoth – that might have been found in a novel by H.P. Lovecraft. In fact Arrokoth meant ‘sky’ in the extinct Powhatan language once spoken by the native people of Chesapeake Bay. The planetesimal was visited by the New Horizons spacecraft two years after it had flown by Pluto (see; Most exotic geology on far-off Pluto, Earth-logs 6 April 2016). It is a small member of the Kuiper Belt of icy bodies. Data collected by a battery of imaging instruments on the spacecraft has now revealed that it has a reddish brown coloration that results from a mixture of frozen methanol mixed with a variety of organic compounds including a class known as tholins – the surface contains no water ice. Arrokoth is made of two flattened elliptical bodies (one 20.6 × 19.9 × 9.4 km the smaller 15.4 × 13.8 × 9.8 km) joined at a ‘waist’. Each of them comprises a mixture of discrete ‘terrains’ with subtly different surface textures and colours, which are likely to be earlier bodies that accreted together. On 13 February 2020 a flurry of three papers about the odd-looking planetesimal appeared in Science.
The smooth surface implies a lack of high-energy collisions when a local cluster of initially pebble sized icy bodies in the sparsely populated Kuiper Belt gradually coalesced under extremely low gravity. The lack of any fractures suggests that the accretions involved relative speeds of, at most, 2 m s-1; slow-walking speed or spacecraft docking (McKinnon, W.B. and a great many more 2020. The solar nebula origin of (486958) Arrokoth, a primordial contact binary in the Kuiper Belt. Science, article eaay6620; DOI: 10.1126/science.aay6620). The authors regard this quiet, protracted, cool accretion to have characterised at least the early stages of planet formation in the Outer Solar System. The extent to which this can be extrapolated to the formation of the giant gas- and ice worlds, and to the rocky planets and asteroids of the Inner Solar System is less certain, to me at least. It implies cold accretion over a long period that would leave large worlds to heat up only through the decay of radioactive isotopes. Once large planetesimals had accreted, however that had happened, the greater their gravitational pull the faster other objects of any size would encounter them. That scenario implies a succession of increasingly high-energy collisions during planet formation.
This hot-accretion model, to which most planetary scientists adhere, was supported by a paper published by Science a day before those about Arrokoth hit the internet (Schiller, M. et al. 2020. Iron isotope evidence for very rapid accretion and differentiation of the proto-Earth. Science Advances, v. 6, article eaay7604; DOI: 10.1126/sciadv.aay7604). This work hinged on the variation in the proportions of iron isotopes among meteorites, imparted to the local gas and dust cloud after their original nucleosynthesis in several supernovas in the Milky Way galaxy during pre-solar times. Iron found in different parts of the Earth consistently shows isotopic proportions that match just one class of meteorites: the CI carbonaceous chondrites. Yet there are many other silicate-rich meteorite classes with =different iron-isotope proportions. Had the Earth accreted from this mixed bag by random ‘collection’ of material over a protracted period prior to 4.54 billion years ago, its overall iron-isotope composition would have been more like the average of all meteorites than that of just one class. The authors conclude that Earth’s accretion, and probably that of the smaller body that crashed with it to form the Moon at about 4.4 Ga, must have taken place quickly (<5 million years) when CI carbonaceous chondrites dominated the inner part of the protoplanetary disc.
See also: Barbuzano, J. 2020. New Horizons Reveals Full Picture of Arrokoth . . . and How Planets Form. Sky & Telescope
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